Metallic sandwich structures are used extensively as load bearing components in many products. Generally, such metallic sandwich structures comprise upper and lower outer skins having an interior structure to provide the desired support, strength and stiffness characteristics. A common sandwich structure comprises two flat steel or alloy sheets with I-beam supports therebetween. The I-beams may be attached between the two metal sheets by a number of fastening systems, i.e., riveting, seam welding, spot welding, etc.
There are various applications where special design criteria must be met and sandwich structural components are used. Trusses for bridges and other architectural structures require strength and durability while maintaining a reasonable strength-to-weight ratio. Likewise, frame components for aircraft must also maintain a high strength and stiffness-to-weight ratios while maintaining the ability to distribute axial as well as shear loads.
Superplastic formation of metallic sandwich structures provides a structure capable of meeting specific design criteria in the most demanding applications. Superplasticity is the ability of certain metals to elongate without necking when deformed under specific temperature and strain rate conditions. Superplastic formation was first utilized in the production of complex shapes. Examples of superplastic formation of metallic sandwich structures are disclosed in U.S. Pat. Nos. 4,217,397 to Hayase et al.; 3,924,793 to Summers; and 3,927,817 to Hamilton. These patents disclose the formation of sandwich structures or panels from three or four layers of sheet metal. One or two sheets of metal are expanded between the outer two sheets of metal by pressurized gas. The inner sheet(s) is capable of being formed into various shapes of inner support structures for the panel. To achieve the structural support shapes within the sandwich structures, seam welds, spot welds and maskants are used to locate connecting points between the outer sheets and inner sheet(s). These assembly requirements are costly and time consuming. Further, the designs in the above referenced patents incorporate necessary flow paths between the expandable sections to insure uniform expansion. At times, such flow paths can seal themselves off, thereby preventing the pressurized gas from fully expanding particular pockets.
Therefore, a method for producing metallic sandwich structures by superplastic deformation is needed that is simple and economical while producing consistent and homogenous structures having adequate strength characteristics.
The present inventive method of using tubes combined with sheets provides a number of benefits over conventional superplastic forming/diffusion bonding (SPF/DB) practice. Curved structures are difficult to manufacture by state-of-the-art SPF/DB methods. Problems of tooling tolerance limitations often result in small but catastrophic forming die mismatch leading to poor, or lack of, diffusion bond between the metal sheets. Unless the metal layers are in intimate contact, there will be little chance for formation of a metallurgical bond. Gaps of a few thousandths of an inch usually lead to failure.
There is always a problem of thickness control with commercial sheet metal products, especially titanium. Tolerances may range up to .+-.5%. When there are variations in the thickness of the metal sheets, the result is the same as if the tooling tolerances are poor which once again can result in poor diffusion bonding. By using the inventive tubes, the problems of tooling tolerance and sheet to sheet thickness variations are essentially eliminated. The tube expands to meet the sheet surface and strongly interfaces with it. It can be likened to an inner tube being inflated within a tire. It completely fills the space. The pressure against the outer walls provides the intimate contact required to achieve a solid diffusion bond.
While conventional multi-sheet superplastic forming/diffusion bonding techniques are easy to demonstrate on flat panels, they are difficult to apply when fabricating the typical curved structures found in aircraft design. The problem is the movement of the sheets relative to each other when forming the curved parts from flat sheets. While each individual sheet could be formed separately and then combined in the final fabrication step, this is very expensive. Not only are the labor and press operation costs multiplied by the number of processing steps needed, there is the added cost of cleaning each of these panels again after forming but prior to the final bonding step to remove any process induced surface contamination that would preclude good diffusion bonding conditions.
Obviously, it is desirable to accomplish both the forming and the diffusion bonding in one pressing. One current approach is to use stop-off coatings to prevent diffusion bonding from occurring where it is not wanted so that the layers of metal can be expanded to form complex structures. Any movement of the sheets which causes misalignment of areas to be bonded causes severe quality problems. Consistent production of reliable structures via this method is difficult.
Another method for production of curved structures is the use of what is known as the "four-sheet" SPF/DB technique. That method uses sheets welded with patterns to regulate the shape and dimension of the expanded structural cells. Unfortunately, the welded center two layers can diffusion bond to each other before the step o of patterned expansion occurs. In addition, the structure formed via this processing method does not result in closed gas pathways as the welded patterns are seldom continuous welds.
Practice of the inventive method of using tubes disclosed herein instead of the welded sheets eliminates the need for welding and will not suffer the problem of closing-off of the gas pressurization pathways. There are structures where active cooling is desired and where the maintenance of separate flow pathways is desired. This inventive method lends itself particularly well to these kinds of structures.